Non-contact electromagnetic locating and tracking systems are well known in the art, with an exceptionally broad spectrum of applications, including such diverse topics as military target sighting, computer animation, and precise medical procedures. For example, electromagnetic locating technology is widely used in the medical field during surgical, diagnostic, therapeutic and prophylactic procedures that entail insertion and movement of objects such as surgical devices, probes, and catheters within the body of the patient. The need exists for providing real-time information for accurately determining the location and orientation of objects within the patient's body, preferably without using X-ray imaging.
U.S. Pat. Nos. 5,391,199 and 5,443,489 to Ben-Haim, which are assigned to the assignee of the present patent application and whose disclosures are incorporated herein by reference, describe systems wherein the coordinates of an intrabody probe are determined using one or more field sensors, such as Hall effect devices, coils, or other antennae carried on the probe. Such systems are used for generating three-dimensional location information regarding a medical probe or catheter. A sensor coil is placed in the catheter and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by a plurality of radiator coils, fixed to an external reference frame in known, mutually-spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
PCT Patent Publication WO 96/05768 to Ben-Haim et al., which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference, describes a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates, so that the position and orientation of the catheter are known without the need for imaging the catheter.
U.S. Pat. No. 6,239,724 to Doron et al., whose disclosure is incorporated herein by reference, describes a telemetry system for providing spatial positioning information from within a patient's body. The system includes an implantable telemetry unit having (a) a first transducer, for converting a power signal received from outside the body into electrical power for powering the telemetry unit; (b) a second transducer, for receiving a positioning field signal that is received from outside the body; and (c) a third transducer, for transmitting a locating signal to a site outside the body, in response to the positioning field signal.
U.S. Pat. No. 4,173,228 to Van Steenwyk et al., whose disclosure is incorporated herein by reference, describes a catheter locating device based upon inducing a signal in a coil attached to the catheter and monitoring the amplitude and phase of the induced signal.
U.S. Pat. No. 5,099,845 to Besz et al. and U.S. Pat. No. 5,325,873 to Hirschi et al., whose disclosures are incorporated herein by reference, describe apparatus and methods in which a radiating element is fixed to a catheter, and the position of the catheter is determined responsive to energy radiated from the element.
U.S. Pat. No. 5,425,382 to Golden, et al., whose disclosure is incorporated herein by reference, describes apparatus and methods for locating a catheter in the body of a patient by sensing the static magnetic field strength gradient generated by a magnet fixed to the catheter.
U.S. Pat. No. 4,905,698 to Strohl, Jr. et al. and U.S. Pat. No. 5,425,367 to Shapiro et al., whose disclosures are incorporated herein by reference, describe apparatus and methods wherein an applied magnetic field induces currents within a coil at the tip of a catheter. Based on these currents, the relative location of the catheter is determined.
U.S. Pat. No. 5,558,091 to Acker et al., which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference, describes a magnetic position and orientation determining system which uses uniform fields from Helmholtz coils positioned on opposite sides of a sensing volume and gradient fields generated by the same coils. By monitoring field components detected at a probe during application of these fields, the position and orientation of the probe is deduced. A representation of the probe is superposed on a separately-acquired image of the subject to show the position and orientation of the probe with respect to the subject.
U.S. Pat. No. 5,913,820 to Bladen et al., whose disclosure is incorporated herein by reference, describes apparatus for locating the position of a sensor, preferably in three dimensions, by generating magnetic fields that are detected at the sensor. The magnetic fields are generated from a plurality of locations and enable both the orientation and location of a single coil sensor to be determined.
Commercial electrophysiological and physical mapping systems based on detecting the position of a probe inside the body are presently available. Among them, CARTO™, developed and marketed by Biosense Webster, Inc. (Diamond Bar, Calif.), is a system for automatic association and mapping of local electrical activity with catheter location.
Electromagnetic locating and tracking systems are susceptible to inaccuracies when a metal or other magnetically-responsive article is introduced into the vicinity of the object being tracked. Such inaccuracies occur because the magnetic fields generated in this vicinity by the location system's radiator coils are distorted. For example, the radiator coils' magnetic fields may generate eddy currents in such an article, and the eddy currents then cause parasitic magnetic fields that react with the field that gave rise to them. In a surgical environment, for example, there is a substantial amount of conductive and permeable material including basic and ancillary equipment (operating tables, carts, movable lamps, etc.) as well as invasive surgery apparatus (scalpels, catheters, scissors, etc.). The eddy currents generated in these articles and the resultant electromagnetic field distortions can lead to errors in determining the position of the object being tracked.
It is known to address the problem of the interference of static metal objects by performing an initial calibration, in which the response of the system to a probe placed at a relatively large number of points of interest is measured. This may be acceptable for addressing stationary sources of electromagnetic interference, but it is not satisfactory for solving the interference problems induced by moving metallic and conductive objects.
U.S. Pat. No. 6,373,240 to Govari, entitled, “Counteracting Metal Presence In A Magnetic Tracking System,” which is assigned to the assignee of the present patent application and is incorporated herein by reference, describes an object tracking system comprising one or more sensor coils adjacent to a locatable point on an object being tracked, and one or more radiator coils, which generate alternating energy fields comprising magnetic fields, in a vicinity of the object when driven by respective alternating electrical currents. For each radiator coil, a frequency of its alternating electrical current is scanned through a plurality of values so that, at any specific time, each of the radiator coils radiates at a frequency which is different from the frequencies with which the other radiator coils are radiating.
The sensor coils generate electrical signals responsive to the magnetic fields, which signals are received by signal processing circuitry and analyzed by a computer or other processor. When a metal or other field-responsive article is in the vicinity of the object, the signals typically include position signal components responsive to the magnetic fields generated by the radiator coils at their respective instantaneous driving frequencies, and parasitic signal components responsive to parasitic magnetic fields generated because of the article. The parasitic components are typically equal in frequency to the instantaneous frequency of the driving frequency, but are shifted in phase, so that the effect at each sensor coil is to produce a combined signal having a phase and an amplitude which are shifted relative to the signal when no field-responsive article is present. The phase-shift is a function of the driving frequency, and so will vary as each driving frequency is scanned. The computer processes the combined signal to find which frequency produces a minimum phase-shift, and thus a minimum effect of the parasitic components, and this frequency is used to calculate the position of the object. Varying the driving frequency until the phase shift is a minimum is described as an effective method for reducing the effect of field-responsive articles on the signal.
U.S. Pat. No. 6,172,499 to Ashe, whose disclosure is incorporated herein by reference, describes a device for measuring the location and orientation in the six degrees of freedom of a receiving antenna with respect to a transmitting antenna utilizing multiple-frequency AC magnetic signals. The transmitting component consists of two or more transmitting antennae of known location and orientation relative to one another. The transmitting antennae are driven simultaneously by AC excitation, with each antenna occupying one or more unique positions in the frequency spectrum. The receiving antennae measure the transmitted AC magnetic field plus distortions caused by conductive metals. A computer then extracts the distortion component and removes it from the received signals, providing the correct position and orientation output.
U.S. Pat. No. 6,246,231 to Ashe, whose disclosure is incorporated herein by reference, describes a method of flux containment in which the magnetic fields from transmitting elements are confined and redirected from the areas where conducting objects are commonly found.
U.S. Pat. No. 5,767,669 to Hansen et al., whose disclosure is incorporated herein by reference, describes a method for subtracting eddy current distortions produced in a magnetic tracking system. The system utilizes pulsed magnetic fields from a plurality of generators, and the presence of eddy currents is detected by measuring rates of change of currents generated in sensor coils used for tracking. The eddy currents are compensated for by adjusting the duration of the magnetic pulses.
U.S. Pat. Nos. 4,945,305 and 4,849,692 to Blood, whose disclosures are incorporated herein by reference, describe tracking systems that circumvent the problems of eddy currents by using pulsed DC magnetic fields. Sensors which are able to detect DC fields are used in the systems, and eddy currents are detected and adjusted for by utilizing the decay characteristics and the amplitudes of the eddy currents.
U.S. Pat. No. 4,791,412 to Brooks, whose disclosure is incorporated herein by reference, describes an article surveillance system utilizing encoded magnetic markers and incorporating a signal processing technique for reducing the effects of large metal objects in the surveillance zone.
U.S. Pat. No. 6,400,139 to Khalfin et al., whose disclosure is incorporated herein by reference, describes a probe tracking system designed to operate in an environment characterized by electromagnetic distortion, such as that caused by eddy currents. The system employs at least one stationary sensor (a “witness sensor”) having a fixed position and orientation near or within a volume of interest. One or more probe sensors are placed on an object to be tracked within the volume, and the output of each witness sensor is used to compute the parameters of a non-real effective electromagnetic source. The parameters of the effective source are used as inputs to the computation of position and orientation as measured by each probe sensor, as if the object were in the non-distorted electromagnetic field produced by the effective source or sources.
U.S. Pat. No. 6,369,564 to Khalfin et al., whose disclosure is incorporated herein by reference, describes a probe tracking system designed to operate in an environment characterized by strong electromagnetic distortion. The system includes at least one source of an AC electromagnetic field, at least one witness sensor measuring components of the electromagnetic induction vector at known locations near or within the volume of interest, and at least one wireless probe sensor placed on the object being tracked. The wireless sensor has a known response or distortion to the electromagnetic field generated by the primary source. Data from the witness sensors are used to locate the probe sensor, treating the probe sensor as a secondary source of the AC electromagnetic field, that is, as a transponder with initially known magnetic parameters. This information is utilized to define coordinates and attitude of the secondary source and, in turn, the position and orientation of the object of interest. Preferably, the probe sensor is an LC-contour tuned to the frequency of the tracker source.
U.S. Pat. No. 6,226,547 to Lockhart et al., whose disclosure is incorporated herein by reference, describes a catheter tracking system that includes a plurality of magnetic field transducers, at least one of which is disposed on the catheter, and others of which are located in/or around the body of the patient and which serve as reference transducers. Magnetic field signals are used to determine the position of the catheter with respect to the reference transducers.
U.S. Pat. No. 5,847,976 to Lescourret, whose disclosure is incorporated herein by reference, describes a method using electromagnetic fields for tracking a mobile system that is placed in a carrier and linked to a magnetic field sensor. The method includes modeling the electromagnetic fields as a function of the coordinates of the sensor, a first field being created by the transmitter, a second field being created by the electrical currents induced in the carrier by the first field, and a third field being created by the electrical currents induced in the mobile system by the first two fields, the magnetic effect of each field being characterized independently of the effects of the other fields by the coefficients of a model thereof. The method further includes real-time computation of the position and orientation of the sensor by using a current measurement of the electromagnetic field at the sensor and by using the models of the fields, the position and orientation of the sensor being defined from a measured field from which the third field is deduced.
U.S. Pat. No. 6,427,079 to Schneider et al., whose disclosure is incorporated herein by reference, describes a remote location determination system that uses splines of magnetic field values to determine location parameters. An automatic calibration technique is described as compensating for any variations in gain in a sensor and related components. Methods for reducing the effects of eddy currents in surrounding conductive objects are described.
U.S. Pat. No. 6,201,987 to Dumoulin, whose disclosure is incorporated herein by reference, describes a tracking system that modifies current patterns applied to its transmit coils in order to compensate for the effect of the eddy currents. The current supplied to the coils is a linear combination of the current needed to create the desired electromagnetic field in the region of interest, and one or more error terms. These terms are determined experimentally during system calibration and are mathematically modeled as a series of exponential functions having a given amplitude and time constant. The error terms in the current applied to the transmit coils are described as canceling the magnetic fields created by eddy currents within the tracking region and as resulting in an actual electromagnetic field which is close to the desired ideal electromagnetic field. The fidelity of the electromagnetic field is described as being further increased by reducing eddy currents within the eddy current inducing structures. This is done by constructing shield coils which are placed between the transmit coil and the eddy current inducing structures. These shield coils are described as creating canceling magnetic fields within the eddy current-inducing structures without substantially altering the electromagnetic fields in the region over which the invasive device is tracked.
U.S. Pat. No. 5,831,260 to Hansen, whose disclosure is incorporated herein by reference, describes a combined electromagnetic and optical hybrid locating system that is intended to reduce the disadvantages of each individual system operating alone.
U.S. Pat. No. 6,122,538 to Sliwa, Jr. et al., whose disclosure is incorporated herein by reference, describes hybrid position and orientation systems using different types of sensors including ultrasound, magnetic, tilt, gyroscopic, and accelerometer subsystems for tracking medical imaging devices.
In the prior art, there is no straightforward, accurate, real-time method that addresses the problem of interference induced in electromagnetic locating and tracking systems caused by the introduction of non-stationary metallic or other magnetically-responsive articles into the measurement environment.